Edexcel IGCSE Chemistry · Spec 2.21
Reactions of Metals with Acids
Covers apply the metal-acid rule to the correct acids, Test safely for hydrogen and Compare magnesium, zinc and iron using a fair method.
Chemistry revision video
Reactions of Metals with Acids
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Explained
Metals with acids, and the rule that has exceptions
Metal plus acid gives a salt plus hydrogen. That is the general rule, and it holds for dilute hydrochloric acid and dilute sulfuric acid, which are the two the specification means.
The salt is named from the acid. Hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates. So magnesium with hydrochloric acid gives magnesium chloride and hydrogen; zinc with sulfuric acid gives zinc sulfate and hydrogen.
The two exceptions worth knowing
Nitric acid does not follow the rule reliably. It is an oxidising agent, so with most metals it produces nitrogen oxides rather than hydrogen, which is why the general rule is quoted for hydrochloric and sulfuric acid only.
Metals below hydrogen in the reactivity series do not react with dilute acids at all. Copper, silver and gold produce no hydrogen, because they are less reactive than hydrogen and cannot displace it. Predicting no reaction is a valid answer, and it needs the comparison with hydrogen to be stated rather than just the claim that the metal is unreactive.
What you see
Bubbles of gas, the metal getting smaller and eventually disappearing, and the mixture warming up, since these reactions are exothermic.
The more reactive the metal, the faster the bubbles. Magnesium fizzes vigorously, zinc more steadily, iron slowly, and copper not at all. That ordering is the basis of the comparison experiment.
Test the gas with a lighted splint. A squeaky pop identifies hydrogen. Both halves matter: a glowing splint tests for oxygen, and saying only that there was a pop, without saying you lit a splint, leaves the method out.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme sets a reaction between excess aluminium powder and sulfuric acid, and asks what is in the beaker at the end. It awards two marks: the excess aluminium, and aluminium sulfate and water.
Beside the second is the instruction that decides it: ignore aluminium sulfate solution unless water is also stated.
That is a precise point about what a solution is. Writing aluminium sulfate solution names one substance and implies the other, and the mark scheme will not take the implication. A solution is a solute and a solvent, and the water was there at the start and is there at the end, so it belongs in the list.
The next part asks why the aluminium was added in excess, and the credited answer is to ensure all the acid reacts, allowing that it neutralises the acid. It then ignores maximum yield, which sounds like a good reason and is not the one being tested.
The question goes on to prove the excess arithmetically, calculating the moles of aluminium from its mass and comparing them with the moles of acid available through the equation's ratio. So excess is not only a practical instruction; it is a claim you can be asked to demonstrate with numbers.
Comparing metals fairly
Change one thing: the metal.
Keep the same volume and concentration of acid, the same mass of metal, the same surface area or form, meaning all ribbons or all powders rather than a mixture, the same temperature, and the same apparatus.
Surface area is the one most often overlooked, and it matters more than it looks. A powdered metal reacts far faster than a strip of the same mass, so comparing powdered zinc with a magnesium ribbon compares two things at once and proves nothing about reactivity.
Measure something. Collect the hydrogen in a gas syringe and record the volume at fixed times, or stand the flask on a balance and record the mass loss. Watching which fizzes most is an observation rather than a result, and it cannot be plotted.
Safety
Wear eye protection. Dilute acids are irritants and the reactions warm up.
Hydrogen is flammable, so keep the lighted splint away from the reaction vessel until you are testing the gas, and collect only small volumes.
Do not use potassium, sodium, lithium or calcium with acid. Those reactions are violent, and the demonstration you have seen with them uses water rather than acid for that reason.
Spec 2.21
What you need to know
- Apply the metal-acid rule to the correct acids
- Test safely for hydrogen
- Compare magnesium, zinc and iron using a fair method
Active recall
Quick check
Answer each question before opening the answer.
Which two dilute acids follow the general metal-acid rule?
Hydrochloric acid and sulfuric acid
What is the test for hydrogen?
A lighted splint gives a squeaky pop
Name two control variables in the comparison
For example acid volume, concentration or temperature
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